Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to cumene hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone.
Another route involves alkylation of benzene with a C4 alkylating agent, such as a linear butene, to produce sec-butyl benzene, followed by oxidation of the sec-butyl benzene to corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and methyl ethyl ketone. An example of such a process, in which the alkylation is conducted in the presence of a catalyst comprising a molecular sieve of the MCM-22 family, is described in U.S. Pat. No. 7,799,956.
Another possible alternative to the conventional Hock process involves the catalytic hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513, in which the hydroalkylation catalyst is a bifunctional catalyst comprising at least one hydrogenation metal and a molecular sieve of the MCM-22 family.
However, one problem in producing phenol via sec-butyl benzene or cyclohexylbenzene is that the oxidation of these higher alkylbenzenes is considerably more difficult than that of cumene. Thus, whereas cumene oxidation is normally conducted in the absence of a catalyst, oxidation of sec-butyl benzene and cyclohexylbenzene typically requires the presence of a cyclic imide catalyst, such as N-hydroxyphthalimide (NHPI), to provide commercially acceptable levels of conversion. Thus, the oxidation proceeds via a free radical chain mechanism and the NHPI increases radical initiation and propagation rates and strongly reduces radical termination rates, thereby improving conversion and selectivity. However, even using NHPI as a catalyst, the selectivity to the desired hydroperoxide decreases with increasing conversion. Thus, the product of the oxidation step typically contains large amounts (of the order of 80 wt %) of unreacted alkylbenzene which must be recycled to ensure acceptable process economics.
The oxidation process also produces a number of undesirable impurities, including certain organic acid species. In addition to hydrolyzing the NHPI, these acid species are known to catalyze cleavage of alkylbenzene hydroperoxides to produce phenol. Although phenol is the desired end-product, its presence during the oxidation process is highly deleterious since, abstraction of a hydrogen atom from phenol, produces the phenoxide radical, which is a stable species having a long lifetime and a slow propagation reaction rate. Thus, phenol inhibits the free radical reactions necessary for the oxidation process to proceed to the hydroperoxide. In the oxidation of cumene in the absence of NHPI, cleavage of the hydroperoxide is prevented by the addition of a base to neutralize any acidic species. However, in the NHPI catalyzed oxidation of alkylbenzenes, addition of base cannot be considered because the base will react with the NHPI and neutralize the O—H bond, which is responsible for the free radical initiation and propagation reactions.
Thus, current methods of oxidizing alkylbenzenes that employ cyclic imide catalysts, such as NHPI, require expensive treatment of the unreacted alkylbenzene to reduce the level of phenol in their recycle streams to very low ppm values. Surprisingly, however, it has now been shown that NHPI will react with phenol under the conditions present in the oxidation reactor to produce an ether species, which has little or no inhibiting effect on the oxidation reaction. As a result it has been found that significantly higher levels of phenol than previously expected can be tolerated in the feed to oxidation reaction, thereby allowing recycle of unreacted alkylbenzene with little or reduced pre-treatment to remove phenol.
U.S. Patent Application Publication No 2011/0037022 discloses a process for producing phenol and/or cyclohexanone by (a) contacting benzene and hydrogen with a first catalyst under hydroalkylation conditions to produce a first effluent stream comprising cyclohexylbenzene, cyclohexane, and unreacted benzene; (b) supplying at least part of the first effluent stream to a first separation system to divide the first effluent stream part into a cyclohexylbenzene-rich stream, a C6 product stream comprising cyclohexane and benzene; (c) contacting at least part of the C6 product stream with a second catalyst under dehydrogenation conditions to convert at least part of the cyclohexane to benzene and produce a second effluent stream comprising benzene and hydrogen; (d) recycling at least part of the second effluent stream to the contacting (a); (e) contacting at least part of the cyclohexylbenzene-rich stream with an oxygen-containing gas in the presence of a third catalyst under oxidation conditions to oxidize the cyclohexylbenzene in the cyclohexylbenzene-rich stream to produce cyclohexylbenzene hydroperoxide; and (f) cleaving cyclohexylbenzene hydroperoxide from (e) to produce phenol and cyclohexanone. Cyclohexylbenzene which is unconverted in the oxidation step (e) is recovered after the cleavage step (f), and is recycled to the oxidation reaction, but according to paragraph [0086], only after the oxidation effluent is treated to remove acids, such as organic acids produced as byproducts of the oxidation reaction and phenolic acids present in the cyclohexylbenzene recycle stream(s).